This invention relates generally to chromium-free welding materials, and in particular to a chromium-free welding consumable and methods employing such consumables for joining or repairing stainless steel base metals, where the weld retains its structural and corrosion properties, even in harsh environments.
Stainless steels or, more precisely, corrosion-resisting steels are a family of iron-base alloys having excellent resistance to corrosion. These steels do not rust and strongly resist attack by a great many liquids, gases, and chemicals. Stainless steels are generally divided into three classes, austenitic, ferritic or martensitic (with a possible austenitic-ferritic duplex class), depending on the predominant microstructural phase. Many of the stainless steels have good low-temperature toughness and ductility, and generally exhibit good strength properties and resistance to scaling at high temperatures. All stainless steels contain iron as the main element and chromium (Cr) in amounts ranging from about 11 to 30 percent, where the presence of Cr in such concentrations enhances corrosion resistance. In the present disclosure, both the atomic symbol (for example, Cr) and its full name are used interchangeably. Additional elements, such as nickel, manganese, silicon, carbon and molybdenum, may be added to impart other desirable properties. Of the three classes, the austenitic stainless steels have the best combination of corrosion resistance, mechanical properties, and weldability, where their corrosion resistance is due at least in part to the high Cr content and nickel additions. An example of an austenitic stainless steel is the American Iron and Steel Institute (AISI) number 304 stainless steel, also called “type 304 stainless steel”, “304 stainless steel” or merely “type 304”. Specific variants of type 304 stainless steel, such as 304L (for low carbon) are often used in naval and related applications.
Stainless steel components are often joined by welding. Consumable filler metals matching or exceeding the Cr content of the base metal have proven to be effective in ensuring that the welds exhibit sufficient corrosion resistance. Existing filler material for welding the various stainless steel base metals, based on Unified Numbering System (UNS) designations include austenitic (UNS Nos. W30810, W30910, W31010, W31610, W31710 and W34710), martensitic (UNS Nos. W41010 and W42010 and ferritic (UNS Nos. S40900 and S43080) formulations. For austenitic stainless steels, such as type 304, the Cr content of the welding consumable is generally between 18 and 20 percent by weight.
During many welding processes, evaporation and oxidation of Cr from the molten weld pool results in the emission of hexavalent Cr that is present in the fumes. In fact, the consumable filler material is typically the major source of welding fumes, sometimes accounting for over 80 percent of the shielded metal arc welding (SMAW) weld metal. Accordingly, the possibility exists for significant generation of hexavalent Cr in the weld fumes. While there are several valence states of Cr (the composition and oxidation state of which depends strongly on the process details such as arc voltage, type of filler material, welding current and the presence of a shielding gas in the welding atmosphere), it is the hexavalent Cr compounds (Cr VI) that are of particular interest, as they are suspected of leading to lung cancer and other health problems. The problem of a Cr VI-rich local atmosphere is exacerbated when the welding is conducted in confined and related spaces lacking adequate ventilation. For example, welding onboard a ship typically involves a manual process using an arc method (such as SMAW or a related electric arc method), which has been shown to generate considerable amounts of fume, up to 0.3 g/min or 8 g/kg of deposit. While these hazardous conditions can be somewhat meliorated by adequate ventilation, such ventilation can be extremely difficult to implement in many situations.
An outgrowth of such significant potential health hazards is that these and other welding operations have been under increased scrutiny recently. For example, the U.S. Department of Labor's Occupational Safety and Health Administration (OSHA) recently decreased the permissible exposure limit (PEL) on Cr VI from 52 to 5 mg/m3 as Cr VI. Manganese (Mn)-bearing fumes are also a concern for manganese toxicity, which affects the central nervous system. As with hexavalent Cr, Mn has been the focus of considerable recent attention, where the OSHA PEL has been set at 5 mg/m3, with the NIOSH REL of 1 mg/m3.
Accordingly, there is a need for developing a consumable for welding austenitic stainless steel that is Cr- and Mn-free to limit the generation of dangerous emission of these metals in the welding fumes.